The adenosine triphosphate (ATP) content was measured independently in separated capsule-epithelium and fibers from whole rabbit lenses, both fresh and after incubation under various combinations of glucose and oxygen deprivation. Lactate production was also measured during aerobic and anaerobic incubations of whole lenses and of monolayers of cultured epithelial cells. The fresh capsule-epithelium contained 3.3 nmoles ATP, whereas the decapsulated lens contained 410 nmoles ATP, a value that was indistinguishable from that of the whole, intact lens. In the presence of glucose, the fibers and epithelium each maintained their respective ATP content under aerobic and anaerobic conditions. In the absence of glucose, the ATP content in each fraction declined with time, but only in the epithelium was the rate of decline of ATP significantly faster in nitrogen than in oxygen. In whole lens, the rates of anaerobic and aerobic lactate production were similar, whereas in the cultured epithelial monolayers, the anaerobic rate was two-fold greater than in oxygen. From this it is concluded that approximately 50% of the ATP of the epithelial cells is derived from oxidative metabolism. A Pasteur response shown here for the first time with the cultured epithelium allows these cells to compensate for the loss of ATP production when mitochondrial oxidation is curtailed. The epithelium does not contribute to the ATP content of the lens fibers under aerobic or anaerobic conditions.